1. Field of the Invention
This invention relates to a crystal article and a method for forming a crystal, particularly to a method for forming a crystal utilizing the difference in nucleation density of the deposited materials according to the kinds of the materials of the deposited surface and a crystal article.
The present invention can be applied to, for example, electronic devices, optical devices, magnetic devices, piezoelectric devices or surface acoustic devices such as semiconductor integrated circuits, optical integrated circuits, magnetic circuits, etc.
2. Related Background Art
In the prior art, single crystal thin films to be used for semiconductor electronic devices or optical devices have been formed by epitaxial growth on a single crystalline substrate.
For example, it has been known to effect epitaxial growth of Si, Ge, GaAs, etc. from liquid phase, gas phase or solid phase on a Si single crystalline substrate (silicon wafer), and also it had been known to effect epitaxial growth of a single crystal of GaAs, GaAlAs, etc. on a GaAs single crystalline substrate.
By use of such semiconductor thin film thus formed, semiconductor devices and integrated circuits, light emission devices such as semiconductor lasers, LED are prepared.
Also, recently, research and development on ultra-high speed transistors by using a two-dimensional electronic gas and ultra-lattice devices utilizing quantum well, has been active. What has enabled these developments the high precision epitaxial technique such as MBE (molecular beam epitaxy) by use of ultra-high vacuum or MOCVD (metal organic chemical vapor deposition method), etc.
In such epitaxial growth on a single crystal substrate, it is necessary to matching the lattice constant and thermal expansion coefficient between the single crystalline material of the substrate and the epitaxial growth layer.
For example, although it is possible to effect epitaxial growth of a Si single crystalline thin film on a sapphire which is an insulating single crystal substrate, there have been the problems such as crystal lattice defect at the interface due to slippage in lattice constants and diffusion of aluminum, which is a component of sapphire, into the epitaxial layer in application to electronic devices and circuits.
Thus, it can be understood that the method of forming a single crystalline thin film of the prior art by epitaxial growth depends greatly on its substrate material. Mathews et al examined combinations of substrate materials and epitaxial growth layers (EPITAXIAL GROWTH, Academic Press, New York, 1975 edited by J. W. Mathews).
Also, the size of the substrate is presently about 6 inches in the case of Si wafer, and enlargement of GaAs, sapphire substrate is more limited.
Further, a single crystalline substrate is high in production cost, and therefore the cost per chip becomes higher.
Thus, for formation of a single crystalline layer capable of forming a device of good quality according to the method of the prior art, there has been the problem that the substrate material is limited to a very narrow range.
On the other hand, research and development have been active in recent years on three-dimensional integrated circuits which accomplish higher integration and multi-function by laminating semiconductor devices in the normal line direction of the substrate.
Also, research and development of large area semiconductor devices are becoming more active year by year such as solar batteries having devices arranged in an array on an inexpensive glass, picture element switching transistors of liquid crystal, etc.
What is common to both of research does not exist the technique of forming a semiconductor thin film on ale non-amorphous insulating material and forming an electronic device such as transistor thereon. Among them, particularly, it has been desired to have a technique for forming a single crystalline semiconductor of high quality on an amorphous insulating material.
Generally, speaking, when a thin film is deposited on an amorphous insulating material substrate such as SiO.sub.2, due to deficiency of long distance order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline (here amorphous film refers to the state where short distance order to the closest extent is preserved but not longer distance order exists, and polycrystalline film refers to a mass of single crystals having no specific crystal direction separated through grain boundaries).
For example, when Si is formed on SiO.sub.2 by CVD method, there is a tendency such that, if the deposition temperature is about 600.degree. C. or lower it becomes amorphous silicon, while if the temperature is higher than that, a polycrystalline silicon with particle sizes of some hundred to some thousand angstroms is obtained. However, the particle size of the polycrystalline silicon varies depending on the formation conditions.
Further, by melting and solidifying the amorphous or polycrystalline film by an energy beam such as laser or rod-shaped heater, a polycrystalline thin film with a grain size of about microns or millimeters has been prepared (Single-Crystal silicon on non-single-crystal insulators, Journal of Crystal Growth, vol. 63, No. 3, October, 1983, edited by G. W. Cullen).
By forming transistors on thin films of various crystal structures thus formed and measuring electron mobilities mobility of about 0.1 cm.sup.2 /V-sec is obtained in amorphous silicon, about 1 to 10 cm.sup.2 /V-sec in a polycrystalline silicon having grain sizes of some hundred angstroms, and mobility to the same extent as in the case of single crystalline silicon in a polycrystalline silicon with large grain sizes by melting solidification.
From these results, it can be understood that there is a great difference in their electrical characteristics between the device formed in a single crystal region within the crystal grain and the device formed as bridging over grain boundary.
More specifically, the deposited film on an amorphous substrate surface has an amorphous or polycrystalline structure, and the device prepared from it is greatly inferior in its performance as compared with the device prepared on the single crystalline layer. For this reason, its is use limited to such as simple switching devices, solar battery, photoelectric converting device, etc.
The methods for depositing crystal layers on an amorphous substrate may be broadly classified into two methods.
One is to use a single crystal (e.g. Si) as the substrate, cover an amorphous insulating material (e.g. SiO.sub.2), remove a part thereof to have the subbing single crystalline surface. The exposed portion is used as the seed crystal for epitaxial growth from gas phase, solid phase or liquid phase and further epitaxial growth in the lateral direction, thereby forming a single crystal region on the amorphous insulating material layer (so called selective epitaxial growth).
The other is to utilize natural nucleus generation without use of a single crystal for the subbing substrate, thereby growing a polycrystalline thin layer.
As described above, there is no long distance order on an amorphous substrate surface as exists on a single crystalline substrate surface, but only short distance order is maintained.
For this reason, the structure of the thin film deposited as such is liable to become amorphous, and the position of grain boundary becomes at best disorderly polycrystalline.
Also, since not only there is no long distance order on the amorphous substrate surface, there is also no anisotropy defining the crystal direction (substrate normal line direction and interplanar direction), it has been impossible to control the crystal direction of the layer thereon.
To summarize the problems of the deposited layer on the amorphous substrate, there are only the controls of grain boundary position and crystal direction.
As to control of the grain boundary position, the present inventors formerly disclosed that the grain boundary position can be determined by previous artificial definition of the nucleation position (Japanese Patent Laid-open Application No. 63-107016), which was named Sentaxy (Selective Nucleation based Epitaxy) (T. Yonehara, Y. Nishigaki, H. Mizutani, S. Kondoh, K. Yamagata, T. Noma and T. Ishikawa, Applied Physics Letters vol. 12, pp. 1231, 1988).
By having fine amorphous Si.sub.3 N.sub.4 localized on amorphous SiO.sub.2 which portion becomes the nucleation surface wherefrom single crystals of Si grow, and through collision against the crystal from the adjoining nucleation surface, a grain boundary is formed to determine the grain boundary position.
However, its crystal direction, particularly the interplanar crystal direction cannot be determined singly, because Si.sub.3 N.sub.4 which is the nucleation surface is amorphous and no anisotropy exists.
On the other hand, in 1978, H. I. Smith disclosed for the first time that by imparting artificial anisotropy with unevenness to the amorphous substrate surface by lithography, the crystal direction of KCl deposited thereon can be controlled, and named this method as Graphoepitaxy (H. I. Smith and D. C., Flanders, Applied Physics Letters. vol. 32, pp. 349, 1978) (H. I. Smith, U.S. Pat. No. 4,333,792, 1982).
Thereafter, it was confirmed that the artificial relief pattern on the substrate surface had influences on the crystal directions in both grain growth of Ge thin film (T. Yonchars, H. I. Smith, C. V. Thompson and J. E. Palmer, Applied Physics Letters vol. 45, pp. 631, 1984) and initial growth of Sn (L. S. Darken and D. H. Lowndere, Applied Physics Letters vol. 40, pp. 954, 1987).
However, in Graphoepitaxy, KCl, Sn were found to have effects of the direction in individual crystals separated at the initial stage of deposition thereof, but for a continuous layer, only crystal growth by laser annealing after deposition of Si (M. W. Gels, D. A. Flanders and H. I. Smith, Applied Physics Letters vol. 35, pp. 71, 1979) and solid phase growth of Ge (T. Yonehara, H. I. Smith, C. V. Thompson and J. E. Palmer, Applied Physics Letters vol. 45, pp. 631, 1984) have been reported.
However, in the case of Si and, Ge, although the direction can be controlled to some extent, the group of crystals are juxtaposed in mosaic shape. Further, randomly positioned grain boundaries different slightly in crystal direction mutually exist between the crystals, whereby no uniform single crystal could be obtained over large area.
The reason is that in addition to the fact that the three-dimensional crystal directions of the crystals are not completely coincident with each other, in the surface relief pattern, the nucleus generation position cannot be controlled. This fact has been found by the present inventors as the result of the studies for long years.